Electric assist steering system

Abstract

An electric assist steering system comprising a first lead screw connected to a steering shaft, a first lead screw nut engaged with the first lead screw, the first lead screw moveable along an axis of the first lead screw, a second lead screw, a second lead screw nut engaged with the second lead screw, the second lead screw connected to the first lead screw nut, an electric motor engaged with the second lead screw nut whereby the second lead screw nut is rotatable by the electric motor, and the first lead screw nut comprises a gear for engaging a steering system pinion.

I claim: 
     
         1 . An electric assist steering system comprising:
 a first lead screw connected to a steering shaft;   a first lead screw nut engaged with the first lead screw, the first lead screw moveable along an axis of the first lead screw;   a second lead screw;   a second lead screw nut engaged with the second lead screw;   the second lead screw connected to the first lead screw nut;   an electric motor engaged with the second lead screw nut whereby the second lead screw nut is rotatable by the electric motor; and   the first lead screw nut comprises a gear for engaging a steering system pinion.   
     
     
         2 . The system as in  claim 1 , wherein the electric motor is connected to the second lead screw nut with a belt. 
     
     
         3 . The system as in  claim 2 , wherein the belt comprises a toothed surface. 
     
     
         4 . The system as in  claim 1 , wherein the belt comprises a multi-ribbed surface. 
     
     
         5 . The system as in  claim 1  further comprising recirculating balls engaged between the first lead screw and the first lead screw nut. 
     
     
         6 . An electric assist steering system comprising:
 a first lead screw;   a first lead screw nut engaged with the first lead screw and moveable along an axis of the first lead screw;   a second lead screw;   a second lead screw nut engaged with the second lead screw;   the second lead screw connected to the first lead screw nut;   an electric motor engaged with the second lead screw nut by a flexible endless member whereby the second lead screw nut is rotatable by the electric motor; and   the first lead screw nut comprises a gear for engaging a pinion.   
     
     
         7 . The system as in  claim 6 , wherein the flexible endless member comprises a belt. 
     
     
         8 . The system as in  claim 6 , wherein the flexible endless member comprises a chain. 
     
     
         9 . An electric assist steering system comprising:
 a first lead screw;   a first lead screw nut engaged with the first lead screw;   a second lead screw;   a second lead screw nut engaged with the second lead screw;   the second lead screw fixedly connected to the first lead screw nut;   an electric motor drivingly engaged with the second lead screw nut whereby a rotation of the second lead screw nut advances the second lead screw; and   the first lead screw nut engagable with a pinion.   
     
     
         10 . The system as in  claim 9 , wherein the electric motor is drivingly engaged with the second lead screw nut by a flexible endless member. 
     
     
         11 . The system as in  claim 10 , wherein the flexible endless member comprises a belt. 
     
     
         12 . The system as in  claim 9  further comprising a plurality of spherical members disposed in a track between the first lead screw and the first lead screw nut. 
     
     
         13 . The system as in  claim 12  further comprising a plurality of spherical members disposed in a track between the second lead screw and the second lead screw nut. 
     
     
         14 . The system as in  claim 11 , wherein the belt comprises a toothed belt. 
     
     
         15 . The system as in  claim 10 , wherein the flexible endless member comprises a chain.